LEAD
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for 2-3 fiours on the water-bath, the liquid is filtered and the precipitate
washed with water containing a little ammonium sulphide and added to
the residue in the porcelain crucible (residue a),, which is then carefully
ignited. The oxides thus obtained are fused with sodium carbonate and
sulphur (see Gravimetric Analysis of Ordinary Bronzes), the fused mass
being extracted with water and the solution reduced with sulphite and
filtered.. The nitrate is added to the alkaline sulphide solution and the
whole used for the determination of the antimony and arsenic (see 3) ; on
the filter the other impurities remain as sulphides.
The precipitate thus obtained is dissolved in nitric acid (i vol. of nitric
acid, D = 1-2, and 2 vols. of water) and the liquid evaporated with a slight
excess of sulphuric acid to eliminate the small quantities of lead still present
and to expel the nitric acid (excess of sulphuric acid is necessary to keep
the bismuth in solution) ; the residue is taken up in a little water, filtered
and treated with hydrogen sulphide. Bismuth, silver, cadmium and copper
sulphides are precipitated (see i), whilst nickel, cobalt, iron, zinc and man-
ganese remain in solution (see 2).
1. Determination of the Bismuth, Silver, Cadmium and Copper.
—The sulphides precipitated with hydrogen sulphide are dissolved in nitric
acid and the solution evaporated in presence of sulphuric acid.
(a) DETERMINATION OF THE BISMUTH. The residue is dissolved in a
little water, almost neutralised with pure sodium hydroxide, treated with
sodium carbonate in slight excess and with a little potassium cyanide and
gently heated. If bismuth is present, a white precipitate is obtained, this
being filtered off, well washed and dissolved in a little nitric acid. The
bismuth is then precipitated with a slight excess of ammonia, the precipi-
tate being washed and dissolved in nitric acid and the solution evaporated
in a tared porcelain crucible ; the residue is ignited gently and weighed :
Bi203 x 0-8965 = Bi.
(6) DETERMINATION OF THE SILVER.*, The .filtrate from the treatment
with sodium carbonate and potassium cyanide is treated with a little more
potassium cyanide and a few drops of sodium sulphide, the silver and cad-
mium being precipitated as sulphides, while any copper present remains
in solution. The precipitate is collected, washed, dissolved in nitric acid
and the silver precipitated as chloride by addition of a few drops of dilute
hydrochloric acid : AgCl X 0-7526 = Ag.
(c) DETERMINATION OF THE CADMIUM. The filtrate from the silver
chloride is evaporated almost to dryness and treated at the boiling point
with sodium carbonate. The precipitate formed is collected, washed with
hot water, dissolved in a little nitric acid, evaporated in a tared porcelain
crucible, gently ignited and weighed: CdO X 0-8754 = Cd.
(d) DETERMINATION OF THE COPPER. The nitrate from the silver and
cadmium sulphides (see 1} is treated with a little sulphuric and nitric acids
and a few drops of hydrochloric acid and evaporated to dryness. The
residue is dissolved in water and the copper determined electrolytically or
gravimetrically.
x The silver is more exactly determined directly by cupellation (see Silver and its
Alloys).H2O = i2Ag + I2HNO3